Update ArrayStack.php

Array object cannot be defined twice, this is a minor typo because it is not the php ArrayObject that is needed in this, but the Stdlib\ArrayObject.

stack call:
Fatal error: Cannot use ArrayObject as ArrayObject because the name is already in use

error is found with the 
skeleton application
doctrine module
doctrine orm module
doctrine dbal

EDIT:
"use ArrayObject;" must be "use Zend\Stdlib\ArrayObject;"